Magnetometer with waveform shaping

ABSTRACT

A detection apparatus discriminates between metallic mines and other buried objects by detecting the depth of the object, the size, the shape and the orientation of the object and the electrical properties of the object. A magnetometer sensor detects objects containing metal located below the surface of the ground. This apparatus has a plurality of parallel, spaced linear conductor sets disposed in proximity to the ground. The conductor sets have varying numbers of individual conductors. An electromagnetic field is imposed in the ground with a dominant spatial wavelength through the conductor elements. A resulting electromagnetic response of the object in the ground to the imposed magnetic field is sensed.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by Contract NumberDAAB07-97-C-J002 from Department of the Army. The Government has certainrights in the invention.

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional ApplicationSerial No. 60/064,808 filed Nov. 7, 1997, U.S. Provisional ApplicationSerial No. 60/043,695 filed Apr. 15, 1997, and U.S. ProvisionalApplication Serial No. 60/034,541 filed Jan. 6, 1997, the entireteachings of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

According to the United Nations, there are over 100 million land minescurrently deployed in more than 60 countries. The mines themselves rangefrom large anti-tank mines to small anti-personnel mines and from allmetal construction to primarily plastic or even wood. Triggeringmechanisms range from direct pressure, to trip wires to magnetic sensorsand fiber optics.

In addition, millions of bomblets were deployed as Cluster Bomb Units(CBUs) during wars and military actions. A significant number of thesefailed to explode and continue to threaten the populations indigenous tothe original combat zones. Being largely constructed of metal,unexploded bomblets are readily detectable with existing hand-held metaldetectors. However, current metal detectors have no way ofdiscriminating an intact bomblet, which may be buried at depths up to 12inches, from a bomblet fragment or other piece of shrapnel or metallicdebris that is near the surface.

The US Army currently has a deployed mine detector called the AN/PSS-12.This is an inductive type detector that utilizes the creation of eddycurrents in a metallic mine to alter the search coil impedance. Thisdetector has served the Army well, but to be reliably detected, minesmust be directly below the search head and must contain some metal.Other methods such as ground penetrating radar, infrared, and X-Ray havebeen investigated to solve the difficult problem of detecting low-metaland no-metal mines.

SUMMARY OF THE INVENTION

This invention relates to detection apparatus and methods which arecapable of discriminating between mines, bomblets and other objectsburied below the surface of the ground by detecting object depths,sizes, shapes, orientations and/or electrical properties. An inductivemagnetometer is best suited to detecting and characterizing metallicobjects; whereas, a capacitive dielectrometer is particularly effectivein detecting and characterizing nonmetallic objects.

In the preferred magnetometer, a plurality of parallel, spaced linearconductor sets are disposed in proximity to the ground. Anelectromagnetic field is imposed in the ground with a dominant spatialwavelength through the conductor sets. A resulting electromagneticresponse of the object in the ground to the imposed magnetic field issensed. The method, in a preferred embodiment, also includes the step oftranslating electromagnetic response into estimates of one or moreproperties of the object based on a modeled response to the spatialwavelength.

In a preferred magnetometer embodiment, the dominant spatial wavelengthhas a length of at least 12 inches. The apparatus also has a rigidconductor element support structure adapted to be scanned across theground.

In a preferred magnetometer, a primary winding has a series of parallel,spaced linear conductor sets driven by a current. The number of parallelconductors in the parallel conductor sets varies so as to shape theapplied magnetic field. The applied field is periodic sinusoidal in apreferred embodiment.

The sensor in a preferred embodiment is an array of secondary windings.At least one of the secondary windings is located between parallelconductor sets of each pair of adjacent parallel conductor sets of theprimary winding. The apparatus may have a second secondary array andprimary winding which is perpendicular to the first set of parallelconductors of the first primary winding.

In a preferred embodiment of the dielectrometer apparatus, an excitationelectrode carried on a sensor face is driven with a varying voltage, anda sensing electrode is carried by the sensor face. A guard electrode ofthe sensor face surrounds the sensing electrode and is at about the samevoltage as the sensing electrode.

A shielding plane is located behind and spaced from the sensor face forblocking unwanted interference in one of the preferred embodiments ofthe dielectrometer apparatus. A guard plate is also interposed betweenthe shielding plane and the guard electrode. A high-impedance buffer isconnected to the sensing electrode to measure the magnitude and phase ofthe floating potential. The sensor face has an area of at least a squarefoot for mine detection but could be used in a smaller form for otherapplications, such as cure monitoring of thin coatings.

In one preferred embodiment of the dielectrometer apparatus, the sensingelectrode has a plurality of elements in a column at different distancesto the excitation electrode. In another preferred embodiment, thesensing electrode has a plurality of elements in a row wherein eachelement is equidistant to the excitation electrodes. The elements may beconnected such that differences in measurements between adjacentelements can be used to detect small spatially abrupt changes in thedielectric properties, and to account for variations in stand-offdistance from the sensor to the soil surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic illustration of a Meandering Winding Magnetometer(MWM);

FIG. 2A illustrates the MWM Sensor;

FIG. 2B illustrates a "Standing Wave" of Magnetic Vector Potential,A_(z), produced by the Dominant Fourier Mode, Corresponding to the A₁Fourier Amplitude;

FIG. 3 shows a Meandering Winding Magnetometer sensor of conductivematerial on a nonconductive substrate;

FIG. 4 an array of Secondary Windings, x₁ through x₈ with CorrespondingAreas of Sensitivity A_(x1) through A_(x8) ;

FIG. 5 illustrate orthogonal arrays of secondary windings, with thecorresponding meandering primary windings;

FIG. 6A illustrates a deep penetration primary winding schematic;

FIG. 6B illustrates a preferred wiring pattern of the deep penetrationprimary winding for shaping the sinusoidal;

FIG. 7A illustrates a two wave length, two orientation MWM detector;

FIG. 7B illustrates a cross section of the sensor over a mine;

FIG. 8A shows a top view of a sensor with the conductor sets all havingcurrent flowing in the same direction;

FIG. 8B illustrates a cross section of the conductor sets and thespatial wavelength;

FIGS. 9A and 9B illustrate conductivity lift-off grids for (a) Aluminumand (b) Carbon Steel;

FIG. 10A illustrates interdigitated electrode dielectrometer (IDED)sensor;

FIG. 10B illustrates a plan view of the IDED sensor;

FIGS. 11A and 11B show two IDED sensors with different electrode spacingillustrating that the spacing effects the sensitive to different depthsinto the material under test;

FIG. 11C illustrates an IDED sensor with an array of sensing electrodes;

FIG. 12 illustrates the IDED characterization of a multiple layermaterial;

FIG. 13 illustrates a single sensing electrode IDED sensor;

FIG. 14 illustrates the equi-potential lines and electric field lines ofsensor cross-section;

FIG. 15A is a cross section of a sensor with multiple sensing elementspositional side-by-side for multiple depths;

FIG. 15B is a perspective view of a sensor with a multiple sensingelements positioned in-line;

FIG. 15C is a cross section of a sensor having a guard plate;

FIG. 16 is a simplified schematic of the detector drive and the feedback sensor;

FIG. 17A shows the scanning results of the sensor shown in FIG. 16 witha plastic mire buried in sand to increasing depths;

FIG. 17B shows the scanning results conducted at different times;

FIG. 17Ca shows the scanning of a mine and a rock;

FIG. 17Cb shows the scanning of a metallic bomblet;

FIG. 18 illustrates a circular center electrode surrounded by acoplanar, concentric electrode, a "Bull's Eye" Sensor;

FIG. 19A illustrates a helicopter deployment of a roadway sensor forrapid minefield breaching;

FIG. 19B illustrates a tape dispensed sensor suitable for path clearing;

FIG. 20 shows a mechanically adjustable wavelength sensor;

FIG. 21A illustrates that the dielectric constants of mine materials andsoils converge at higher frequencies and diverge at lower frequencies;

FIG. 21B illustrates that conductivity is frequency dependent for soilsand plastic mine materials and much higher at high frequencies;

FIG. 22A illustrates that the computed sensor capacitance change(decrease) due to buried plastic layer in dry sand; and

FIG. 22B illustrates that the computed change (increase) in sensorcapacitance from simulated metal mine (layer construct) in dry sand.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to two new sensing capabilities that arecomplementary and both field deployable and supportable. The firstsensing capability is magnetoquasistatic, an inductive sensor, whichwill be referred to as Meandering Winding Magnetometer (MWM). The secondsensing capability is an electroquasistatic, capacitive sensor whichwill be referred to either as an Interdigitated Electrode Dielectrometer(IDED) for periodic constructs or as a dielectrometer for non-periodicconstructs. The sensors individually have certain capabilities todetermine the depth, material type, size and orientation of a subsurfaceobject as described below. The synergy of the two sensors allows furthercapability.

The principal surface considered is that interface between air andground, wherein the ground is a mixture of one or more of dirt, sand,rock, moisture and other such materials. A subsurface object isgenerally an object which is located within 2 feet of the surface andparticulary within 12 inches.

Meandering Winding Magnetometer (MWM) Sensor

The magnetoquasistatic sensing capability using a Meandering WindingMagnetometer (MWM) will be described first. The MWM comprises ameandering primary winding, with one or more secondary windings such asthe meandering secondary on each side of the primary as illustrated inFIG. 1. The MWM is essentially a planar transformer, in which theprimary winding is inductively coupled with the secondary windingthrough the neighboring material.

The secondary windings, which meander on opposite sides of the primary,are connected in parallel to reduce capacitive coupling and to maintainsymmetry as illustrated in FIG. 2A. The winding spatial wavelength isindicated by λ. A current, i₁, is applied to the primary winding and avoltage, v₂, is measured at the terminals of the secondary windings.

The shape of the MWM windings produces a spatially periodic magneticfield as shown in FIG. 2B. The spatial periodicity of the field is a keyattribute of the MWM and is the principal reason it can be modeled withsuch accuracy. The MWM continuum models permit precise determination ofdepth and material properties for detected objects.

The MWM is tailored such that the magnetic vector potential produced bythe current in the primary winding can be accurately modeled as aFourier series summation of sinusoids in Cartesian (x,y,z) coordinates.The tailoring is described in further detail in U.S. Pat. No. 5,453,689titled "Magnetometer Having Periodic Winding Structure and MaterialProperty Estimator" which issued on Sep. 26, 1995, the entire contentsof which are incorporated herein by reference.

In the magneto-quasistatic regime, the MWM primary winding produces asinusoidal "standing wave" magnetic vector potential. The spatialwavelength of this standing wave is determined by the MWM primarywinding geometry and is independent of the input current temporalfrequency. The fundamental Fourier mode wavelength is equal to thephysical, spatial wavelength of the MWM primary winding, as shown inFIG. 2B. The uniform standing wave field produced by the MWM sensormaintains its shape over a significant footprint area. Thus, for MWMswith multiple periods (i.e., more than four) a subsurface object, suchas a bomblet, will produce the same MWM response at all locations withinthe MWM footprint (i.e., <1/2 wavelength from the edge of the sensingregion).

The MWM sensors can be fabricated in several embodiments. These can haveeither multiple periods, a single period (i.e., only one period of asine wave is produced by the field shaping primary), or a fraction of aperiod (e.g. half). While the embodiments will be described with respectto preferred embodiments for a particular size range, such descriptionsare not meant to limit particular sizes to particular embodiments.

One embodiment of sensors is fabricated by deposition and selectiveremoval of a conducting material on a thin film nonconducting substrateas seen in FIG. 3. This printed conducting material is considered awire. This method of sensor construction allows the sensor to be verythin and of very low mass.

An alternative embodiment of sensor is to use a series of wires coiledinto a desire pattern. This embodiment is a preferred embodiment forsensors having a surface area over a square foot. In a preferredembodiment the sensor is approximately 32 in.×24 in., with only oneprimary winding period.

In certain embodiments of the over square foot array, arrays ofsecondary windings elements provide spatial resolution of indications onthe order of an inch. This effectively maps the individual metalliccomponents under the MWM footprint, permitting discrimination of anintact bomblet from bomblet fragments, shrapnel, or other metallicdebris. This array construct also permits the use of multiple turn(coil) sensing elements (also called secondary windings) in the form ofelongated coils. Thus, a large wavelength drive is used to provide deeppenetration, while multiple sensing elements are used to provide highspatial resolution.

FIG. 4 shows an array of secondary windings confined to a single plane.The individual windings in this array are designated x₁ through x₈. (Theassociated primary winding is not shown in this figure.) Ideally, eachindividual secondary winding would be sensitive only to conductingmaterial in an elongated area, shown as A_(x1) through A_(x8), in FIG.4. In practice, however, the secondary winding elements will be somewhataffected by objects located outside of these regions, as well. Thus, amore complex computation will be required to create accurate images ofdetected objects.

Windings can be stacked for increased output. For example, the arrayshown in FIG. 4 can be repeated in successive layers. The output of eachillustrated coil, such as X₁ crosses over to the input of the next lowercoil such that a stack of turns X₁ result in a spiral coil. In orderthat the array follows the model closely, the thickness between coilplanes should be small relative to the wavelength of the array.

FIG. 5 illustrates an orthogonal combination of two secondary windingarrays. These arrays, along with the associated meandering primarywindings also shown in the figure, are stacked in one embodiment of thebomblet discrimination sensor to provide the mechanism for locating andsizing detections.

The typical required depth of penetration for the detection ofsubsurface weapons such as land mines and bomblets is six to twelveinches from the surface. A rule of thumb for the depth of penetration ofthe MWM fields is that the maximum field penetration is approximatelyone half wavelength from the sensor winding plane. The 24 inch by 24inch MWM, with the orthogonal sensing arrays described above and asshown in FIG. 5, has a wavelength of 6 inches and has a depth ofpenetration of only 3 inches.

For those situations where a deeper penetration of detection isrequired, a deep penetration winding as shown schematically in FIG. 6Amay be used. The deep penetration winding is composed of multipleparallel conductors, half of which carry current in one direction whilethe other half carry current in the opposite direction. By controllingthe current carried by each conductor, this winding design can produce asingle period of a sinusoidal magnetic field over the MWM sensing regionwith a spatial wavelength of 32 inches in the preferred embodiment for aparticular bomblet's type. This design can also provide shorterwavelength excitations, i.e., 12 inches, by electronically changing thecurrent directions without changing the physical winding dimensions.This permits the use of multiple spatial wavelength excitations andperformance of spectral analysis in space (instead of time) to improveclutter suppression and detection probability. This multiple wavelengthinterrogation approach is described in greater detail in U.S. Pat. No.5,015,951 titled "Apparatus and Methods for Measuring Permeability andConductivity in Material using Multiple Wavenumber MagneticInterrogations" which issued on May 14, 1991, the entire contents ofwhich are incorporated herein by reference.

The deep penetration winding permits detection of bomblets to a depth ofup to 16 inches from the surface, one half its maximum spatialwavelength. The secondary array, as discussed above with respect to thesix inch wavelength MWM described in FIG. 4, can be used with either thesix inch wavelength MWM primary or with the deep penetration primarywinding design. The secondary array permits better spatial resolutionthan the use of larger sensing elements.

By changing the connections of the parallel conductors, either throughhard wired connections or switching, the wave length of the sensor arraycan be varied from a maximum illustrated in FIG. 6A, where a first halfof the conductors conduct current in one direction and a second halfconduct in the opposite direction, to a minimum as illustrated in FIG.2B.

The use of multiple MWM spatial wavelengths provides depth informationon detected objects. For example, when the six inch MWM primary isexcited, objects deeper than three inches will not be detected. Thus, abomblet buried at ten inches will only be detected when the deeppenetration primary is excited. This provides a clear capability todiscriminate between small metallic objects near the surface andbomblets buried far below the surface.

A preferred wiring pattern of the deep penetration primary winding forshaping the sinusoidal wave is shown in FIG. 6B. The primary winding hasa series of parallel, spaced sets 200 of linear conductors for receivingcurrent. Each conductor set has at least one wire. The number ofparallel wires in each set increases from 1 to 2 to 3 and then back downto 1 before the center line. The number of parallel wires in a set thenprogresses back to 3 and back down to 1 in the second half wavelength.

Those wires on the left side of the centerline of the sensor have thecurrent flowing up the page as seen in FIG. 6B. Those wires on the rightside of the page have the current flowing down the page. The varyingnumbers of wires in each set and the flow of the current results in adeep penetrating electromagnetic waveform that has a single wavelengthequal to the size of the sensor.

An array of secondary windings 202 is illustrated in FIG. 6B. At leastone secondary winding is located between each adjacent pair of parallelconductors of the primary winding. In a preferred embodiment, the wiresof the primary winding and the secondary windings are insulated metalconductors.

The sets of parallel wires in one embodiment are equally spaced. Inanother embodiment the spacing of the wires is also varied to shape theapplied magnetic field.

A multiple wavelength MWM sensor configuration is shown in FIG. 7A. TheMWM sensor stack shown includes two different winding spacings (λ₁, λ₂)and two different orientations. Since the spacing of the MWM windingsdetermines the depth of penetration, this permits a real-timedetermination of depth and an estimate of aspect ratio as discussedbelow. FIG. 7B illustrates the sensor of FIG. 7A over a land mine.

FIGS. 8A and 8B show a sensor that has a plurality of conductor setsformed of wire. Each conductor set has the same number of wires. Thecurrent in each conductor set 90 is flowing in the same direction, upthe page as seen in FIG. 8A and into the page as seen in FIG. 8B. Theresult is a uniform field in the central sensing region where spatialwavelength is essentially infinite. The spatial waves have beenpredominately described above as having a sinusoidal shape. It isrecognized that other spatial waves may be desired in certain instancessuch as saw tooth waves, square waves, pulsed, and impulse. The width ofthe spatial wave can also vary. The wave can likewise increase ordecrease as the wave progress over the sensor.

The MWM sensor is driven by an AC current and its response is measuredby an impedance analyzer. In a preferred embodiment, a circuitboard-level, multi-frequency impedance instrument having a range of 250KHz-2.5 MHZ is used. The response is compared to the continuum models.The sensor response which is in the terms of impedance phase andmagnitude is converted into material properties or conditions ofinterest, such as conductivity and proximity. Proximity is the averagedistance between the winding plane and the surface of the conductingburied object.

In addition to permitting precise determinations of material properties,the MWM modeling software also incorporates methods to identifyoperating conditions that provide maximum sensitivity and selectivity(the ability to measure two or more properties independently), withoutrunning extensive experiments. The identification of operating conditionis described in further detail in U.S. Pat. No. 5,015,951 titled"Apparatus and Methods for Measuring Permeability and Conductivity inMaterials Using Multiple Wavenumber Magnetic Interrogations" whichissued on May 14, 1991 and a U.S. patent application Ser. No. 08/702,276titled "Meandering Winding Test Circuit" and filed on Aug. 23, 1996, theentire contents of which are incorporated herein by reference.

Once an object is detected, the depth below the surface, and the sizeand shape of the object need to be ascertained in order to determine howto proceed. For example, if it is determined that the object is anintact mine or bomblet, the object needs to be marked, disarmed orremoved. However, if it is determined that the object is fragment ordebris, the object could be left.

One of the keys to discrimination will be determination of depth of anobject of unknown size. For example, a small metal object located nearthe surface may be detected by more than one sensing element, as would alarge object located far from the surface. Thus, to differentiatebetween these objects, depth information is required. Using model-basedMWM grid measurement algorithms, the depth of a metallic object detectedby an individual secondary (sensing) element can be determined. Also, anobject's size and shape can be determined by combining information aboutthe proximity of the object to the sensing elements with the number andlocation of the sensing elements that detected the object. Additionalinformation may also be provided by the magnitude and phase of thedetection signal at different input current frequencies, and fordifferent sensing element orientations. This additional information isused to increase detection sensitivity and to improve cluttersuppression.

MWM can discriminate the location and the properties of the object byusing some or all of the following approaches: (1) Spatial imaging; (2)Grid measurement algorithms; (3) Spectral analysis, spatial andtemporal; and (4) a scanning or roving sensing element. The rovingsensing element can be oriented either in a parallel or perpendicularplane to the MWM primary.

Spatial imaging approaches utilize the orthogonal array of secondarywindings to provide spatial resolution and permit discrimination andanalysis of multiple indications. The array output is operated upon by alogic module which applies the above analysis and is then used to drivea visual output display, discussed below, or an auditor signal to theoperator. The visual display will provide the interface with the systemoperator.

Grid measurement algorithms permit the integration of impedancemeasurement data at multiple frequency, multiple winding spatialwavelengths, and multiple lift-offs (by moving the MWM sensor or using aroving sensing element). This integration is used in conjunction withthe array calibration discussed below. The result is a multi-dimensionalclutter suppression and bomblet identification algorithm that willprovide robust, reproducible, and high confidence bomblet discriminationcapability. It provides real-time (fast) measurements, enabled by tablelook-up from stored measurement grids.

Measurement grids are tables produced by the continuum models of the MWMand in a preferred embodiment are graphically displayed. The measurementgrids are used to convert the MWM impedance magnitude and phasemeasurements into material properties or material proximity. Thereal-time table look-up process is described in U.S. patent applicationSer. No. 08/702,276 which is titled "Meandering Winding Test Circuit"which was filed on Aug. 23, 1996, the entire contents of which isincorporated by reference.

The grid measurement approach allows for detection and discrimination ofvarious objects including various types of landmines and bombletscontaining metal and other unexploded ordinance. The measurement gridsalso provide a unique tool for rapid field calibration of sensingarrays.

To generate measurement grids, the material conductivity (or otherproperty of interest) is first estimated using calibration standards orvalues from the literature. (This estimate merely serves to define thegeneral region of interest in which to run the models to generatepredicted sensor response.) The continuum models of the MWM then predictsensor response, in terms of phase and magnitude, using the selectedranges of conductivity and proximity (lift-off). This type of grid iscomposed of lines of constant lift-off intersecting lines of constantconductivity. These grids are generated off-line and then provide areal-time (fractions of a second) measurement capability in the field.

FIGS. 9A and 9B illustrate measurement grids for aluminum and carbonsteel. Note that the lift-off lines for aluminum are practicallyperpendicular to those of carbon steel. This offers a very directapproach for discriminating between steel and aluminum: simply vary thesensor lift-off (i.e., move it up and down relative to the ground) andobserve the orientation of the lift-off line. This will provide a simplebut effective filter for eliminating aluminum objects, such as discardedcontainers, and other nonferrous metals from further considerationduring bomblet discrimination. The system operator would only berequired to move the sensor head vertically over an indication, whilethe software compares the lift-off response for the detected object tothe stored lift-off response (in the measurement grid) generatedoff-line and calibrated for an intact bomblet.

Spectral analysis approaches involve operating MWMs of several spatialwavelengths at various excitation frequencies to provide moreinformation about the sensed volume. The additional information frommultiple grids obtained at different MWM spatial wavelengths and atmultiple input current temporal frequencies can be used to determine thematerial type, size, depth and case thickness of the sensed object, aswell as to further define and constrain the bomblet or landmine"signature", and improve clutter suppression and bomblet detectionperformance.

The scanning or roving sensor involves maneuvering a movable secondarywinding (or electrode in the case of the dielectrometers) within thefield of a fixed primary winding. This is an alternative approach to theuse of multiple sensing elements to provide spatial imaging fordiscriminating intact bomblets.

The combination of MWM design and operational features with the gridmeasurement approach provides redundant paths to solution of the bombletdiscrimination problem. Table 1 lists the system features and theinformation produced by each to support bomblet discrimination andclutter suppression. Each one of the four key attributes required tofully characterize an intact bomblet (size, shape, depth and material)can be generated by at least two of the system design or operationalfeatures.

                  TABLE 1                                                         ______________________________________                                        Bomblet discrimination features produced by MWM-                              Array System                                                                  SYSTEM OR OPERATIONAL                                                         FEATURES             DISCRIMINATOR                                            ______________________________________                                        Secondary winding    Size, shape, depth                                       array*                                                                        Multi-frequency      Material, size, depth                                    measurements*                                                                 Multiple spatial     Depth                                                    wavelengths*                                                                  Multiple proximity   Material, size, depth                                    measurements*                                                                 Rotating sensor head Shape                                                    ______________________________________                                         *When combined with measurement grids.                                   

The information gathered by the sensor needs to be displayed ordisclosed to the user quickly and efficiently. The goal is to processthe data and present the result in an unequivocal way that requiresminimal operator interpretation. This will greatly reduce the trainingrequired for the user or operator.

A sensor array output may be located directly above the sensor. Thedisplay could be LED, LCD or other display device. The display is drivenby two sensing elements, one in each of the orthogonal arrays. An LED isilluminated if both its associated sensing array elements detect ametallic object. An alternative embodiment to the sensors locateddirectly above the sensor is a display located closer to the users.

Dielectrometer Sensor

While the magnetoquasistatic detection using the Meandering WindingMagnetometer is capable of determining location, shape and orientationof metal, the MWM sensor is not capable of detecting plastic or othernon-conducting objects within the ground. There exist many land minesthat have very low metal content. Even if the MWM sensor was able todetect the metal, the size, shape and orientation of the metal detectedwould not allow the user of the sensor to ascertain whether the metalwas or was not part of a land mine.

The second sensing capability, the dielectrometer, capacitive sensor iscapable of detecting subsurface plastic as described below. Thedielectrometer, capacitive sensor senses the dielectric properties ofthe material.

The dielectric properties of a material can be described by twoparameters, the permittivity and conductivity. The permittivitydescribes the displacement current density produced in the material byan applied electric field, whereas the conductivity describes theconduction current density. The dielectric properties of materials varysignificantly and can provide a means for identification of materials.

It is convenient to represent the complex permittivity of a material asε*=ε'-jε", where ε' is the real part and ε" is the imaginary part of thecomplex permittivity. The real part is the dielectric constant of thematerial (ε'=ε); whereas, the imaginary part (ε"=σ/ω whereσ=conductivity and ω=angular frequency of the electric field) describesthe power dissipation in the material (loss). The dielectric spectrum ofa material is a representation of its complex permittivity, expressed asa function of frequency. The dielectric spectrum provides a uniquesignature of a material in a particular state.

Classical dielectrometry extracts information about the state of amaterial construct from its dielectric spectrum. The application of asinusoidally varying potential of complex magnitude V and angularfrequency ω=2Πf results in the flow of a terminal current with complexamplitude I, whose magnitude and phase is dependent on the complexpermittivity ε* of the material.

A capacitive sensor 100 in one preferred embodiment is an interdigitatedelectrode dielectrometer (IDED) sensor 102 such as presented by Melcheret al. in U.S. Pat. No. 4,814,690, "Apparatus and Methods For MeasuringPermittivity in Materials". The IDED 102 utilizes a pair ofinterdigitated electrodes 104 and 106 to produce a spatially periodicelectric field. A typical arrangement of such electrodes is shown inFIG. 10A.

The electrodes are adjacent to the material of interest with aninsulating substrate and a ground plane on the other side of thesubstrate. One of the two electrodes, 104, is driven with a sinusoidallyvarying voltage, v_(D), while the other, 106, is connected to ahigh-impedance buffer used to measure the magnitude and phase of thefloating potential, v_(S). The periodicity of the electrode structure isdenoted by the spatial wavelength λ=2p/k, where k is called thewavenumber.

A plan view of the IDED sensor is seen in FIG. 10B. The drivenelectrode, an excitation electrode 104, has a plurality of fingers 108.The other electrode 106, the electrode connected to the high-impedancebuffer and referred to as a sensing electrode, has a plurality offingers 110. The fingers of the two electrodes are interdigitated on thesensor face, such that fingers of the first electrode and the secondelectrode alternate across the sensor face.

One inherent benefit of the IDED structure is that the coupling of theapplied field into the medium can be achieved from a single surface.Dielectric measurements of thin films, for example, can be performedwithout having to deposit a metal electrode to the exposed side of thesample.

The depth of sensitivity of the sensor is determined by the electrodespacing. The electric scalar potential in the dielectric above thesensor in FIG. 10A obeys Laplace's equation and in a Cartesian geometrywith a linear lossy dielectric the solutions are of the form:

    Φ=Φ.sub.0 e.sup.-kx [A sin ky+B cos ky]

This indicates a general property of solutions to Laplace's equation: ifthe excitation is periodic in space, the potential decays in theperpendicular direction with a penetration depth into the unknowndielectric equal to the spatial wavelength of the spatially periodicexcitation.

An IDED is sensitive to material within a distance (from the electrodeplane) of up to one third to one half of the spacing between electrodes.Sensors with different electrode spacing will consequently be sensitiveto different depths into the material under test, even when operated atthe same excitation frequencies as illustrated in FIGS. 11A and 11B. Forheterogeneous media, spatial profiles of dielectric properties can bedetermined using multiple wavelength sensors, as each wavelength has aunique penetration depth into the heterogeneous dielectric.

The magnitude and phase of the measured signal from an IDED sensordepend on the sensor geometry and the dielectric properties of thematerials in proximity to the sensor. The sensor geometry and thedielectric properties of the materials determine the complex admittanceof the sensor, i.e., the ratio between the current and the voltagebetween the two electrodes.

FIG. 11C illustrates an IDED sensor 112 with an array design. Thesensing electrode 116 is formed of a multiple of elements 118.Individual elements can be selected to locate the position of theunderground object.

The admittance can be calculated from the complex surface capacitancedensity C, defined as C=ε*E_(x) /Φ, where E_(x) is the electric field inthe x direction. The spatially periodic potential Φ is derived from thevoltage between the electrodes. The current in the sensing electrode canbe determined by integrating the quantity ε*E over the area of theelectrode. Therefore all the information about the material structure iscontained in the surface capacitance density.

When a single uniform layer whose thickness is much greater than theelectrode wavelength is present, C can be derived by solving Maxwell'sequations in the electroquasistatic case to be C=e*k, where k is thespatial periodicity wavenumber.

When more than one layer is present, such as when an air gap existsbetween the sensor and the soil surface, the surface capacitance densityat the electrode surface is calculated by sequentially deriving C at allmaterial interfaces, beginning with the topmost layer as illustrated inFIG. 12. For every layer, if C is known at the upper surface, it may becalculated for the lower surface (also from Maxwell's equations) as afunction of e* and thickness, d, of that layer. Using this approach, theair layer between the sensor and the soil surface is taken into account.

The multiple-wavelength approach to property profiling uses IDEDs withdifferent spatial wavelengths to measure complex permittivity variationswith depth at a particular location on the component surface. Eachsensor element of the multiple-wavelength IDED produces a measurementwhich corresponds to a depth of material that is proportional to thewavelength of that particular element. The element with the shortestwavelength will respond to the dielectric properties of the materialclosest to the surface, whereas the longer-wavelength elements will besensitive to the material below that as well. Thus, the complexpermittivity profile of the material can be determined from measurementsmade with multiple-wavelength IDEDs. This is described in further detailin U.S. Pat. No. 4,814,690 titled "Apparatus and Methods for MeasuringPermittivity in Materials" which issued on Mar. 21, 1989, the entirecontents of which are incorporated herein by reference.

The ability to independently vary the applied frequency and the spatialwavelength of the electrodes allows one to measure both the temporal andthe spatial frequency response of the material. The temporal response,or dielectric spectrum, is obtained by varying the excitation frequency,and the spatial response is obtained by varying the spatial wavelengthof the sensor. Because the temporal (ω) and spatial (k) domains areindependent, this technique has been referred to as the `imposed ω-k`approach to dielectrometry.

One of the features that differentiates this approach from classicaltechniques utilizing single wavelength structures is the fact that theheterogeneity of the material under test can be deduced independentlyfrom the temporal frequency response. This can be achieved by performingvariable spatial wavelength measurements at the same temporal frequency.The spatial distribution of the dielectric properties can thus bedetermined without making assumptions about the nature of the material.This additional freedom allows an unconstrained evaluation of thephysical mechanisms that govern the dispersive nature of the dielectricproperties.

Quasistatic indicates that the frequency of excitation is sufficientlysmall such that the propagation of electromagnetic radiation over thearea of interest is approximately instantaneous and thereforeapproximately obeys a simplified version of Maxwell's Equations in whicheither electric or magnetic fields are of primary interest. In the caseof this sensor it is the electric fields which are of primary interestand it is the coupling of electrodes through these electric fields whichis termed capacitance. Since the sensor uses this coupling to probe formaterials of varying dielectric properties such as landmines, the termcapacitive has been used to describe the sensor.

An alternative embodiment to the interdigitated electrode dielectrometer(IDED) sensor is a sensor that has a single sensing electrode, or asingle location for a sensing electrode, and excites only one period ofthe electric field. This design is more appropriate for non-portablesensors. A multiple wavelength (periodic) version of this sensor couldbe used for vehicle mounted applications.

The basic single sensing electrode sensor 120 design as illustrated inFIG. 13 consists of two excitation electrodes 122, a sensing electrode124, a guard electrode 126 and a shielding plane 128.

The excitation electrodes 122 are driven by a high voltage source whichis typically sinusoidal (500V peak in experiments). Electric field linesemanate from the excitation electrodes and fringe through the half-spaceabove and below the face of the sensor, terminating on the shieldingplane 128, guarding electrode 126 and sensing electrode 124. In thepreferred approach, the primary sensing electrode 124 is held at avoltage potential equivalent to that of the shield and guard, which istypically a ground reference, while the current required to maintainthis sensing electrode voltage is measured. Alternatively, the sensingelectrode could be allowed to float, its voltage being detected. Keepingboth the sense electrode and shield/guard electrodes at identicalvoltages effectively eliminates the capacitive coupling between theseelectrodes. Such coupling can result in signal attenuation andsensitivity loss, since it is the coupling between the sense electrodeand excitation electrodes that is of interest.

The ratio of excitation voltage to the current flowing to and from thesensing electrode, also known as the transimpedance, is then used as thesensor output. The output is compared with the response from both finiteelement and analytical models of the sensor and its surroundings todetermine material or geometric properties of the surroundings. Theoutput during scanning is compared with the output with no buriedobjects present when used to detect changes in the surroundings overposition or time.

The overall structure is driven by the desire to induce dielectricpolarization in materials which are not locatable directly betweenelectrodes, but rather materials which are in a half-space regionseparated from all electrodes in the adjacent half-space. In order toaccomplish this, fringing electric fields are setup by electrodes heldat two different voltage potentials and placed in the plane separatinghalf-spaces. The use of two excitation electrodes at the same potentialadds a degree of symmetry to the fields, while placing the sensingelectrode at the center eliminates disturbances from unwantedinterference as a result of the protection from the shielding plane. Theuse of a single excitation electrode permits deeper sensor penetrationwith the same size footprint. In terms of electric field distribution,the sensing electrode and guard shield can be viewed as a singleelectrode since they are at the same voltage potential. The spatialdistribution of the fringing fields is then primarily determined by theexcitation electrode and sense/guard shield electrode size and positionin the plane of the sensor face.

From closed form 2-D Laplacian solutions for electric fields withperiodic boundary conditions it is known that the electric fieldintensity will decay into the half-space possibly containing thelandmine. It is also known that boundary conditions on potential havinglower spatial frequencies will result in a slower rate of decay ofelectric field intensity with distance from the electrode plane. Thisfact is utilized in the aperiodic structure by separating excitation andguard/sense electrodes until practical sensor size limitations arereached, thereby increasing the low frequency spatial spectral contentof the boundary potential at the sensor's face. The gap betweenelectrodes and electrode widths have been chosen so that the potentialat the boundary approximates that of a single period of a sinusoid inorder to minimize higher spatial harmonics which will cause anundesirably faster decay of the relative electric field intensity.Placing the shield plane too close to the face of the sensor also tendsto create higher order harmonics and is therefore placed as far aspractical from the sensor face. All of these efforts are aimed atincreasing the relative electric field intensity as deep as possibleinto the half-space being probed. However, having sufficient fieldintensity at a desired probing depth into the half-space is necessarybut not sufficient in being sensitive to the materials located there.Further attention must be given to the design of the sense electrode andguard.

Referring to FIG. 14, the electric field distribution of andielectrometer sensor illustrates several factors which the placement ofelements increase sensitivity. Electric fields exist in both thehalf-space being probed and the region between the guard/sensor and theshield plane. The electric field lines between the guard/sensor and theshield plane primarily terminate on the side of the guard electrodeopposite the half-space being probed. The termination of these fieldlines contribute to the total current flowing to and from theguard/sense electrode; however, this portion of the current isinsensitive to the region being probed. The placing of the guardelectrode parallel to the sense electrode and opposite the region beingprobed eliminates the current flow to the sense electrode on the sidebetween the sensor face and the shield plane, resulting in a greatersensitivity of the current to the region being probed.

Still referring to FIG. 14, it can be seen that field lines with theshallowest penetration terminate on the outer edges of the guard/senseelectrodes, while field lines with the deepest penetration terminate inthe middle of the electrodes. It should also be noted that the densityof field lines terminating on the electrode tends to decrease withincreasing depth of penetration. This is due to the fact that theelectric fields inherently decay with distance from their chargesources. As a result, the sensitivity of the measured current flowing inand out of the sense electrode is inherently biased toward nearproperties rather than deep properties. In order to counter this effectthe sense electrode has been reduced in width so that only deeppenetrating field lines terminate on it, increasing the sensitivity ofthe measured current to deep effects. The guard electrode replaces theparts of the sense electrode, which were reduced in order to maintainthe low spatial spectral frequency components.

Additional imaging capability may also be achieved by further breakingthe original single sensing electrode 124 in separate parts as shown inFIG. 15A, giving further information about depth of objects in thehalf-space being probed. This sensor 120 utilizes a single column of aplurality of sensing side-by-side elements 132. In preferredembodiments, there are three or five elements. As can be seen from FIG.14, the center sensing electrode of FIG. 15A senses the longest anddeepest spatial half wavelengths, while end electrodes sense shorter,shallower half wavelengths.

Breaking the sensing electrode up into separate elements along what hasbeen the depth of the cross-section as depicted in FIG. 15B, allows forimaging of the half-space being probed. This sensor 120 utilizes asingle row of four sensing elements 134 surrounded by the guardelectrode 126. A pair of drive electrodes are located on either side.The four sensing electrodes can be connected differently such that threeoutputs are produced which are proportional to differences in adjacentelectrodes. In a preferred embodiment numerous (e.g. 20 sensing elementswill be used in a row to increase image resolution). Without the sidesensing elements of the previous embodiments, this sensor does notinclude air gap compensation capability. When the output of each elementis directly used as information in building an image, results similar toscanning a single element will be obtained. The array of elements excelswhen utilized with additional circuitry, which differences themeasurements from adjacent or alternating elements. Differencing theelements allows for additional sensitivity to small, but spatiallyabrupt (with respect to the spacing of the elements being differenced)changes in the dielectric properties in the half-space, as is the casewhen searching for objects such as landmines. With the sensor stationarya one-dimensional image is formed by numerically integrating themeasured differences after their conversion from analog signals todigital values. By scanning the array in a direction perpendicular tothe line of array elements two-dimensional images may be formed bycombining the one-dimensional image at each position of the scan. Hereincorporating an absolute measurement (i.e., not differential) of one ormore of the elements at each scan position can be useful in accountingfor variations in the sensor lift-off when scanning over a surface.Additional information from electrodes sensitive to properties atvarious depths as described in the previous section may also beincorporated for improved object discrimination and three-dimensionalimaging. A full two dimensional array combining the features of FIGS.15A and 15B may also be provided.

FIG. 15C illustrates an alternative embodiment to the sensor of FIG.15A. The sensor has a guard electrode 126 which surrounds theside-by-side elements 132 on the same plane. A separate guard plate 140overlies, underlies as seen in FIG. 15C, the sensing electrode and theguard electrode. The guard plate is electrically connected to the guardelectrode.

The input-stage of a circuit for measuring the current flowing in andout of the sense electrode, while maintaining a virtual groundpotential, is shown in FIG. 16. It includes an electrometer gradeoperational amplifier in a current integration configuration. Thecurrent integration configuration is preferred over the commontransimpedance amplifier consisting of a resistor in the feedback looprather than a capacitor. This is primarily due to the relatively smallcurrent being measured and the relatively large resistor which would berequired. The resistor in the feedback loop serves only to bleed chargeoff of the integration capacitor to avoid saturation from staticelectric fields and amplifier bias currents. The RC time constant of R1in parallel with C1 is set such that the break frequency is less thanthe excitation frequency, but the time constant is fast enough to allowa fast decay of accumulated charge from stray static fields at thesensor electrode when scanning.

In the case of an array of elements used in differential mode, eachelement would utilize one of the previously described circuits. Theoutput from pairs of circuits from adjacent or alternating elementswould then be fed into a common difference amplifier providing a singleoutput for each pair of electrode.

A test of the sensor shown in FIG. 15B was conducted. A land minespecimen of interest (M14) was buried to the desired depth in the sandbed. For this test a scanning mechanism for the test bed was created.The sensor data acquisition system was initiated and the scan motor wasenergized. The data acquisition system automatically records thedifferential sensor outputs (the difference between adjacent sensingelements).

The data produced during a scan consists of an array of data points,four elements wide by about 200 elements long. In the preferredembodiment numerous (e.g. 20 or more) elements will be used and rapidscanning (e.g. 1 ft/sec.) will be provided. This data can be used tocreate an image by connecting data points of equal value with contourlines, as a topographic map uses contour lines to connect points ofequal elevation. Intermediate values can be developed by interpolationto increase contour line density.

FIG. 17A illustrates the results of a sequence of scans conducted withthe inert M14 mine buried at increasing depths in the sand bed. The datashown is raw data that had not been processed to remove air gap effects.Future signal processing efforts would substantially improve the qualityof mine images produced by the capacitive sensing array. In the top scanthe top of the mine is at the surface of the sand. In the second scandown, the top of the mine is buried to a depth of 1 cm below thesurface, and so on.

The mine can be clearly seen at up to 2 cm depth in the images produced.Not only are the sensor capacitance values significantly affected by thepresence of the mine, but also the gradient from unaffected values isquite steep. It is recognized that pattern recognition techniques suchas edge detection can be used to discriminate mines from clutter. Thisdata was taken at a single frequency. Once a target is detected,multiple frequencies are used for false alarm rejection anddiscrimination/identification of mines. For example, dielectricspectroscopy can permit discrimination between plastic and rocks.

FIGS. 21A and 21B show the dielectric constant and conductivityvariations with frequency for several materials. This shows that a greatdeal of information is available in the quasistatic frequency rangeunder 100 MHZ compared to the 1-10 GHz range that ground penetratingradar operates.

Tests were also conducted to evaluate system repeatability over time.FIG. 17B illustrates the results of scans conducted approximately threehours apart. These scans were conducted with the M14 mine at a depth of1 cm. Sensor and system response to the mine and the surrounding sandbed is extremely repeatable.

Several scans were conducted to show the clutter suppression capabilityand metallic object detection. A buried rock was seen to be readilydistinguishable from the plastic M14 mine and the metallic object (adime) was also easily detected by the capacitive sensor. FIG. 17Caillustrates the difference between the M14 mine and the rock. The rockis the area at the 47 cm mark, whereas the M14 mine is at about the 36cm mark and on the centerline of the scan. Also, a dime was placedbetween the rock and the landmine. The dime was also detected. FIG. 17Cbillustrates the ability of the capacitive array to detect metallic minesand UXO. The area at the 37 cm mark is an intact bomblet buried to adepth of 1 cm. As would be expected from the inclusion of a material ofhigh dielectric constant, such as the metallic bomblet, within thesensed volume of sand, the sensor capacitance is increased. Also, thedime can be detected easily at approximately 1 cm depth using thecapacitive array.

FIGS. 22A and 22B show the IDED responses for metal and plastic atvarious depths. Thus, metal and plastic can be easily differentiated.For example, if the soil dielectric constant is above that for theplastic then the metal and plastic response will go in oppositedirections.

Dielectric spectroscopy can be preformed to identified material types.The types of spectroscopy that can be preformed include derivativespectroscopy, ratio spectroscopy and other common types.

Dielectrometry measurements using long wavelength IDEDs can easilydiscern planar plastic and metal structures below the surface of dry orwet sand, and can be used to detect land mines without ground contact.However, an alternative embodiment may be more effective in certaininstances. An alternative embodiment is an electrode arrangement whichutilizes a circular center electrode surrounded by a coplanar,concentric electrode, a "Bull's Eye" sensor as illustrated in FIG. 16.

An axisymmetric computer simulation of the response of this sensor to arealistic nonmetallic mine was run. The mine was modeled as a 4 cmdiameter cylinder, 4 cm thick, made of ABS and rubber, with two smallmetal components (firing pin and fuze), buried in dry sand. Thesedimensions and materials are representative of the Chinese Type 72 APmine. A 2.5 cm sensor stand-off distance was used. Sensor capacitance isshown in Table 2. Wet sand further improves measurement sensitivity byproviding values of conductance as well as capacitance. Since ourexisting computer simulation software can only solve planar geometriesfor AC conduction problems, the planar analog to the "Bull's Eye"geometry and cylindrical mine was simulated at 10 Hz. The calculatedcapacitance and conductance per unit length are listed in Table 3.

                  TABLE 2                                                         ______________________________________                                        Computed "Bull's Eye" Sensor Response to Buried                               Nonmetallic Mines in Dry and Wet Sand.                                                Dry Sand  Wet Sand (10 Hz)                                            Mine Depth                                                                              Capacitance Capacitance                                                                             Conductance                                   (cm)      (pf)        (pf/m)    (S/m × 10.sup.-14)                      ______________________________________                                        0         5.276       19.4      1.51                                          2.5       5.363       21.4      2.18                                          5.0       5.386       21.4      2.03                                          7.5       5.395       21.3      1.95                                          10        5.398       21.3      1.91                                          No Mine   5.401       21.3      1.83                                          ______________________________________                                    

This preliminary analysis of the "Bull's Eye" electrode arrangementindicates fair capacitance response to nonmetallic mines close to thesurface in dry sand, but poor response at increasing burial depths. Inwet sand, however, the sensor conductance remains sensitive to themine's presence down to 10 cm depth, as was seen in the planar layeranalysis of lossy soil in the previous section. The lossy soil has asubstantial ε" as defined earlier.

Combined MWM and Dielectrometer Approach

Both the MWM and the dielectrometer sensors individually can detect andcharacterized land mines and bomblets that existing technology could notdetect or, if detected, could not characterized. However, the results ofboth sensors produces details that were not possible individually. Table3 shows the characteristics of both.

                  TABLE 3                                                         ______________________________________                                        Summary of the materials and characteristics for                              MWM and IDED.                                                                          Mine Constituents Placement                                                                             plastic                                                                             soil                                                high    low   non-  (low  (clay,                               Sensor                                                                              Potential                                                                              ferrite ferrite                                                                             ferrous                                                                             or no-                                                                              loan,                                type  attribute                                                                              metals  metals                                                                              metals                                                                              metal)                                                                              etc.)                                                                              sand                            ______________________________________                                        MWM   detection                                                                              .check mark.                                                                          .check mark.                                                                        .check mark.                                                                              .check mark.                                                                       .check mark.                          depth    .check mark.                                                                          .check mark.                                                                        .check mark.                                                                              .check mark.                                                                       .check mark.                          shape    .check mark.                                                                          .check mark.                                                                        .check mark.                                                                              .check mark.                                                                       .check mark.                          identifica-                                                                            .check mark.                                                                          .check mark.                                                                        .check mark.                                                                              .check mark.                                                                       .check mark.                          tion                                                                    IDED  detection                                                                              .check mark.                                                                          .check mark.                                                                        .check mark.                                                                        .check mark.                                                                        .check mark.                                                                       .check mark.                          depth    .check mark.                                                                          .check mark.                                                                        .check mark.                                                                        .check mark.                                                                        .check mark.                                                                       .check mark.                          shape    .check mark.                                                                          .check mark.                                                                        .check mark.                                                                        .check mark.                                                                        .check mark.                                                                       .check mark.                          identifica-                  .check mark.                                                                        .check mark.                                                                       .check mark.                          tion                                                                    ______________________________________                                    

The MWM comprises a meandering primary winding, with a secondary windingon each side of the primary. The MWM is essentially a planartransformer, in which the primary winding is inductively coupled withthe secondary winding through the neighboring material. The IDED, on theother hand, consists of two separate, but coplanar, electrodes. Whereasthe MWM can be considered a transformer within a single plane, the IDEDcan be considered a parallel plate capacitor within a single plane.

While no single tool will detect and identify all subsurface objects,the MWM (or single period deep penetration array) and the IDED (orimproved single period design described earlier) sensors will locatedand identify a majority of mines and bomblets for humanitarian deminingproblems.

In one embodiment the deep penetration MWM-Array might be used to detectmetal, then the single period dielectrometer might be used to determineif the metal is surrounded by plastic.

It is recognized that the MWM-Array system and the IDED-Array system canhave features that can be used to augment the work of various othertools. For example, to support the use of an airknife for bombletremoval following a detection, the MWM-Array Bomblet DiscriminationSystem could include a means of marking the exact bomblet location. Oncea bomblet has been identified, the sensor array could be maneuvered toposition the bomblet to a particular part of the array, for example thecenter. The system would be equipped with a means of dispensing spraypaint or another environmentally friendly marker directly to the exactbomblet location.

While the sensors discussed above have been individual sensors, it isrecognized that a Local Positioning System (LPS) could be used tocoordinate large area scanning by teams of field operators and to mapand record buried ordinance and clutter locations. In view of therelative lightweight of the sensors, a light-weight mat array couldcover large arrays, several hundreds of feet, without explodingencountered landmines, such as illustrated in FIGS. 19A and 19B. Thelight-weight mat has a flexible sheet, such as a durable flexible fabricor composite material, that retains the conductors in proper position.

It is recognized that the wavelength can be varied by placing theelectrodes on an adjustable face plate as illustrated in FIG. 20. Boththe driven electrodes and the sensing electrodes are attached to anaccordion or scissors frame, which can expand and contract. The movingof the driven electrodes and the sensing electrodes closer or furtherapart results in mechanically varying the wavelength.

EQUIVALENTS

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims. Those skilled in the artwill recognize or be able to ascertain using no more than routineexperimentation, many equivalents to the specific embodiments of theinvention described specifically herein. Such equivalents are intendedto be encompassed in the scope of the claims.

What is claimed is:
 1. An apparatus for detecting electromagneticproperties comprising:a primary winding having a series of parallel,spaced linear conductor sets for receiving a current, the primarywinding having at least one conductor associated with each parallelconductor set and having different numbers of conductors associated withparallel conductor sets to shape a spatial magnetic waveform generatedby the primary winding; and an electromagnetic sensor comprising atleast one secondary winding for sensing a resulting electromagneticresponse.
 2. The apparatus of claim 1 wherein the waveform is asinusoid.
 3. The apparatus of claim 2 wherein the sensor comprises anarray of secondary windings, wherein at least one of the secondarywindings is located between parallel conductors of each pair of adjacentparallel conductor sets of the primary winding.
 4. The apparatus ofclaim 3 wherein a plurality of secondary windings are disposed in twodimensions.
 5. The apparatus of claim 3 further comprising a secondsecondary array and a second primary winding which are orientedperpendicular to the conductors of the first primary winding.
 6. Theapparatus of claim 1 wherein the waveform is one period of a sinusoid.7. The apparatus of claim 1 wherein the linear conductors on one side ofa center line of the primary winding have current flow in one directionand the linear conductors on the other side of the center line havecurrent flow in the opposite direction.
 8. The apparatus of claim 7wherein the sensor comprises an array of secondary windings, wherein atleast one of the secondary windings is located between parallelconductor sets of each pair of adjacent parallel conductor sets of theprimary winding.
 9. The apparatus of claim 1 wherein a plurality ofsecondary windings are disposed in two dimensions.
 10. The apparatus ofclaim 1 where the primary winding comprises a conduction lead whichpasses back and forth in plural conductor sets and combines with itselfto form the different numbers of conductors.
 11. The apparatus of claim10 where the conductor lead alternates between half cycles of thewaveform.